Thermal imaging is the art of photographing an object in the infra-red portion of the spectrum, by sensing the thermal radiation which is supplied by the object itself. Some applications of thermal imaging do not need to take into account the transmissivity of the atmosphere or the presence of noise caused by specularly reflected infra-red radiation from the sun. Applications such as detecting heat leaks in an inadequately insulated home, nondestructive testing of materials, engine analyses, or body scanning for medical diagnostic purposes all fall into this category because they can either be performed indoors or at night, so that sunlight is not a complicating factor, or they can be performed at such close range that high atmospheric transmissivity is not required. But other applications, principally those involving airborne or satellite imaging, must be conducted outdoors and at a wavelength capable of penetrating a substantial intervening thickness of atmosphere. These applications include military reconnaissance to locate camouflaged enemy operations, and surveys to identify stressed plants, predict crop yield, detect forest fires, perform wildlife inventories, estimate soil moisture and evaporation rates, or measure thermal inertia of geologic rock formations.
There are only two "windows" in the transmission spectrum of the atmosphere in which thermal imaging from aircraft or space is feasible. One is the wavelength band from 3 to 5 .mu.m and the other is the wavelength band from 8 to 13 .mu.m. High sensitivity in the 8-13 .mu.m band can only be achieved with cryogenically cooled detectors, such as Hg-Cd-Te material operating at 77.degree. K. Thermoelectric cooling is not sufficient to reach such low temperatures. Therefore, sensitive thermal imaging in the 8-13 .mu.m band always requires a supply of liquid nitrogen to be carried in bulky and heavy Dewar containers as part of the imaging equipment. In addition, certain precautions are necessary because of the hazards associated with liquid nitrogen. Also, a user in a remote place is forced either to carry along a large extra reservoir of liquid nitrogen, or to travel regularly to a distant source to replenish the supply.
Sensitive thermal imaging in the 3-5 .mu.m band, on the other hand, is possible without the use of liquid nitrogen. Cooling of the detector (such as PbSe) is necessary to some degree, but only down to 195.degree. K., which can be reached using a thermo-electric cooler. Thermal imaging in this band at night has been done successfully. But during the day, such thermal imaging is severely disturbed by the presence of specularly reflected sunlight of the same wavelength.
The purpose of this invention is to provide instrumentation that is capable of analyzing radiation outgoing from the object into two components, the reflected sunlight component and the thermal emission component, in order to record high-quality thermal images in bright sunlight, while using only light-weight, electronically cooled equipment.
What is needed to accomplish this objective is some way of discriminating specularly reflected sunlight from thermal radiation, when both are in the 3-5 .mu.m wavelength band. Techniques have been described in the literature for discriminating between two different kinds of radiation, specular and diffuse, the basis of the discrimination being a difference in degree of polarization. But until now it appears that no one has devised a way of separating thermal from reflected radiation. The work described in the literature, moreover, has been confined to the visible portion of the spectrum, where there is no significant amount of thermal radiation at ordinary temperatures.